1. Field of the Invention
The present invention is directed to apparatus and method for drilling a well into earth formations lying below a body of water, wherein the wellhead equipment of the well is positioned below the surface of the water. The well is drilled from a floating drilling vessel, with a riser conduit connecting the vessel drilling equipment to the wellhead assembly.
2. Description of the Prior Art
An increasing amount of offshore deepwater exploratory well drilling is being conducted in an attempt to locate oil and gas reservoirs. These exploratory wells are generally drilled from floating vessels. As in any drilling operation, drilling fluid must be circulated through the drill bit in order to cool the bit and to carry away the cuttings. This drilling fluid is normally returned to the floating vessel by means of a large diameter pipe, known as a riser, which extends between the subsea wellhead assembly and the floating vessel. The lower end of this riser is connected to the wellhead assembly which is generally adjacent to the ocean floor, and the upper end usually extends through a centrally located hull opening of the floating vessel. A drillstring extends downward through the riser into earth formations lying below the body of water, and drilling fluids circulate downwardly through the drillstring, out through the drilling bit, and then upwardly through the annular space between the drillstring and riser, returning to the vessel.
As the water depths for these drilling operations continue to increase, the length of the riser and subsequently its unsupported weight also increases. Since the riser has the same structural buckling characteristics as a vertical column, riser structural failure may result if compressive stresses in the elements of the riser exceed the metallugical limitations of the riser material. Two separate mechanisms are typically used to avoid the possibility of this cause of riser failure.
Riser tensioning systems are installed on board the vessel, which apply an upward force to the upper end of the riser, usually by means of cable and sheave mechanisms connected between the vessel and the upper elements of the riser.
Buoyancy or ballasting means may also be attached to the submerged portion of the riser. These usually are comprised of syntactic foam or individual ballast tanks formed on the outer elements of the riser section. The ballast tanks are capable of being selectively inflated with air from the floating vessels air compression equipment. Both of these buoyancy devices create upwardly directed forces in the riser, compensating for the compressive stresses created by the risers weight, and thereby preventing riser failure.
Since the riser is fixedly secured at its lower end to the wellhead assembly, the floating vessel will move relative to the upper end of the riser due to wind, wave, and tide oscillations normally encountered in the marine environment.
This creates a problem because the stationary riser located within the hull opening of the oscillating vessel can contact and damage the vessel, unless it remains safely positioned within the hull opening. For this reason motion-compensating equipment incorporated with the riser tensioning system is used to steady the riser within the hull opening, and usually takes the form of hydraulically actuated cable and sheave mechanisms connectably engaged between the upper riser elements and the vessel structure, and a flexible coupling located in the riser adjacent the vessel's hull. This equipment allows the vessel to heave, surge, and sway, without contacting the upper elements of the riser.
Directional positioning thrusters, in addition to the normal maneuvering system of the vessel, compensate for normal current and wind loading, and prevent riser separation due to the vessel being pushed away from the wellhead location.
All of these systems, however, can only prevent riser compressive failure, separation, or contact with the vessel during normal sea state conditions.
The capacity of these systems is exceeded with winds typically over 35 to 40 mph and/or swells over a height of 25 feet. Above these values, further measures need to be taken to prevent riser and vessel damage.
The riser may be disassembled in sections and stowed on the floating vessel's deck, but the time required for this operation usually exceeds the warning time given by an oncoming storm system.
The riser may be disconnected from the wellhead assembly and thereby become suspended from the vessel. The vessel with the suspended riser then may remain in the vicinity of the wellhead assembly, or the vessel may attempt to tow the riser out of the path of the approaching storm. In either situation, once the riser's lower element disconnected from the wellhead assembly, the riser becomes a vertically orientated submerged vessel with its own oscillatory characteristics, or "bobbing" tendencies, typically different than those of the supporting vessel. When the vessel and riser heave upward, due to the vessel riding the crest of the wave, the riser may continue upwards while the vessel is falling downwards in a subsequent wave trough. This uncontrolled upward riser movement and subsequent downward movement through the center of the hull opening can exceed the allowable vertical movement and load capacity of the normal motion compensating and tensioning equipment, causing severe damage to the vessel and riser, with attendant risk to crew and vessel. Further means need to be developed to prevent the riser from this uncontrolled upward and downward motion within the vessel's hull opening.